Meanwhile, heat skillet with drippings over medium heat. Crack eggs into skillet; cook until whites are set but yolks are still runny, 2 to 3 minutes. Set 6 pancetta slices and 2 eggs atop each of 2 cheese toasts. Top eggs with onion, Parmesan, and basil, then remaining bread slices, cheese side down. Cut sandwiches in half and serve.